In a liquid crystal panel, a polarizing plate is used to control optical rotation of light beams that pass through liquid crystals. Conventionally, in such a polarizing plate, a polarizing plate obtained by dying a polymer film, such as a polyvinyl alcohol or the like with iodine or a dichromatic dye and stretching the film in one direction has been widely used. However, there has been a problem that the aforementioned polarizing plate is poor in heat resistance and light resistance depending on the kind of the dye or the polymer film. Moreover, there is a drawback that the polarizing plate has a considerable great thickness.
In contrast, a method for forming a polarizing film by casting a coating solution including organic dyes exhibiting lyotropic liquid crystallinity on a substrate, such as a glass plate or a polymer film and the like to form a polarizing film by orienting the organic dyes is known. The organic dyes exhibiting lyotropic liquid crystallinity form supramolecular aggregates in the solution, so that the long axis direction of the supramolecular aggregates is oriented in a flowing direction when flowing after applying shearing stress onto the coating solution including this (JP 2006-323377 A). Such a polarizing film employing organic dyes does not need to be stretched and is easy to have a greater width because of no shrinkage in a width direction by stretching. Further, the polarizing film is expected to have potential because the thickness can be reduced significantly.
Conventionally, organic dyes having a sulfonate group where a sulfonic ion (—SO3−) is connected to a monovalent cation (e.g., Li+) have been used for a portion of polarizing films in which organic dyes are aligned. Such polarizing films are poor in water resistance because a sulfonate group is ionized to be dissolved in water. In contrast, it is possible to obtain a water-resistant polarizing film which is insoluble or has poor solubility in water by substituting the monovalent cation of the sulfonate group for a divalent cation which is insoluble in water to be water-resistant treated (JP 11-21538 A).
In a conventional process for producing a water-resistant polarizing film, however, the orientation degree of organic dyes was deteriorated when the aforementioned water-resistant treatment was performed, which resulted in a problem of a decrease in dichroic ratio.
Conventional processes for producing a water-resistant polarizing film suffered from the deterioration of the orientation degree of organic dyes and dichroic ratio. It is an object of the present invention to provide a process for producing a water-resistant polarizing film which is free from the deterioration in the orientation degree of organic dyes and the dichroic ratio.